In an announcement, Dr. Reddy's said, "Given that this action stays approval of our ANDA until the case is resolved; it is not surprising that Pfizer has sued us. We are, however, confident of our position and are working to resolve the matter as quickly as possible."

Lipitor remains to be a best seller for Pfizer, which in 2008 drove back Ranbaxy's effort to launch a copycat version to market.

But, the company has witnessed a fall in Lipitor's sales.

In the first nine months of 2009, Lipitor's global sales stood at around $8.3 billion as against $9.3 billion in the corresponding period of 2008.

"Pfizer will be irreparably harmed if Dr Reddy's is allowed to sell the drug in the US market," Pfizer lawyers said in federal court.

A Dr Reddy's representative said, "We are confident of our position and are working to resolve the matter as quickly as possible."